At present, some motor vehicles are equipped with “hands free” access. This means that the authorized user of the vehicle no longer needs a key in order to open the doors and other openings (bonnet, boot, etc.) of his vehicle. He has instead an identification badge (or remote control) with which the electronic system of the vehicle interacts.
In order to command the opening of a door, for example, the user approaches the door handle. A capacitive presence sensor situated in the handle detects the presence of the user. This sensor is connected to the electronic computer ECU (Electronic Control Unit) of the vehicle and sends it a presence detection signal. The electronic computer of the vehicle has previously identified the user as being authorized to access that vehicle or, alternatively, it carries out this identification after receiving this signal. In order to do this it sends, by the intermediary of an LF (Low Frequency) antenna, an identification request to the badge (or to the remote control) carried by the user. This badge in response sends its identification code to the vehicle electronic computer by means of RF (Radio Frequency) waves. If the electronic computer recognizes the identification code as being the one authorizing access to the vehicle, it initiates the opening of the door. If, on the other hand, the electronic computer has not received the identification code or if the identification code is incorrect, the opening does not take place.
Such a capacitive sensor is constituted by a capacitor electrode Ce (see FIG. 1) integrated in the door handle (not shown).
When the user's hand M approaches the door handle, that is to say when the user approaches the electrode in FIG. 1, the capacitance Ce of the electrode integrated in the handle increases by a value ΔCe. This variation ΔCe of the capacitance Ce is measured by a measuring device D. If the value of the variation ΔCe exceeds a threshold, this results in the validation of the detection of the presence of the hand M close to the door handle. In fact, this signifies that the user's hand M is sufficiently close to the handle and that he is requesting access to the vehicle.
According to the prior art, the device D for measuring the variation of the capacitance Ce shown in FIG. 1 comprises:                a power supply voltage Vdd,        a capacitance Ce, generally in the form of an electrode, having a voltage Vce across its terminals,        charging means 101 and of discharging 102 the capacitance Ce, which carries out a predetermined number Nc of charging and discharging cycles of the capacitance Ce,        comparison means 200, in the form of two comparators,                    a first comparator 201, comparing the voltage Vce across the terminals of the capacitance Ce with respect to a first reference value Vref−, and            a second comparator 202, comparing the voltage Vce across the terminals of the capacitance Ce with respect to a second reference value Vref+,                        controlling 300 means of the charging means 101 and of the discharging means 102 which activate the charging means 101 and the discharging means 102 of the capacitance Ce according to the result of the comparisons carried out by the comparison means 200 (201, 202) and according to a logic described in detail below,        a counter 400 which measures the time tmes2 necessary for the measuring device D to carry out the predetermined number Nc of cycles of charging and discharging the capacitance Ce,        calculating means 500, which calculate a time variation Δt between this time tmes2 and a previously measured time tmes1, Δt=tmes2−tmes1, the time variation Δt representing the variation ΔCe of the capacitance Ce.        
The capacitance Ce is successively charged and discharged by the charging means 101 and by discharging means 102 according to a predetermined number Nc of charging and discharging cycles. According to the prior art, the charging means 101 are, for example, a first current source G1 connected to the power supply voltage Vdd, associated with a first switch SW1 connected to the capacitance Ce. When the first switch SW1 is closed (state 1), the capacitance Ce is electrically connected to the first current source G1 which charges it with current i. The discharging means 102 are, for example, a second current source G2 connected to ground, associated with a second switch SW2 connected to the capacitance Ce. When the second switch is closed (state 1), the capacitance Ce is connected to ground through the second current source G2 which discharges it with a current i. When the first switch SW1 is closed (state 1), the second switch SW2 is open (state 0) and vice-versa.
The voltage Vce across the terminals of the capacitance Ce therefore varies according to the state of the first switch SW1 and of the second switch SW2, that is to say depending on whether the capacitance Ce is being charged or discharged.
This voltage Vce is compared with a first reference value Vref− and with a second reference value Vref+ by the first and second comparators 201 and 202 respectively. A value of a first output S1 of the first comparator 201 is a function of the result of the comparison with the first reference value Vref−. For example, the first output S1 takes the value 0 when Vce>Vref− and it takes the value 1 when Vce<Vref−. Similarly, a value of a second output S2 of the second comparator 202 is a function of the result of the comparison with the second reference value Vref+. For example, the second output S2 takes the 0 when Vce<Vref+ and it takes the value 1 when Vce>Vref+.
The first and second outputs S1 and S2 are connected to the inputs of the control means 300. The control means 300 are typically, according to the prior art, a logic circuit of the of the synchronous flip-flop type, also called an “SR flip-flop”. The first output S1 is connected to a first input, input S, of the control means 300 and the second output S2 is connected to a second input, input R, of the control means 300. An output Q of the control means 300 provides a control signal SL of the first switch SW1 and the second switch SW2, of value 0 (state 0: switch open) or 1 (state 1: switch closed) depending on the values S1 and S2 received at the inputs S and R.
The control means 300 activate the charging 101 or discharging 102 means, that is to say more precisely the first SW1 and second SW2 switch according to the values received at the R and S inputs in order to discharge or charge the capacitance Ce.
Charging and discharging cycles of the capacitance Ce are shown in FIG. 2. FIG. 2 shows the variation of the voltage Vce across the terminals of the capacitance Ce during the chargings C+ and during the dischargings C− of the capacitance Ce as a function of time t. As shown in FIG. 2, during a charging C+, the voltage Vce increases, from the first reference value Vref− to the second reference value Vref+. During a discharging C−, the voltage Vce decreases from the second reference value Vref+ to the first reference value Vref−. The voltage Vce therefore oscillates between the first reference value Vref− and the second reference value Vref+.
As the value of the charging current i is equal to the discharging current i, the charging time is equal to the discharging time of the capacitance Ce. This (charging or discharging) time will be called t1. The time of a cycle T1 comprising a charging and a discharging is therefore equal to:
                              T          ⁢                                          ⁢          1                =                              2            *            t            ⁢                                                  ⁢            1                    =                                    2              *              Ce              *                              (                                                      Vref                    +                                    -                                      Vref                    -                                                  )                                      i                                              Equation        ⁢                                  ⁢                  (          1          )                    where:    T1: time of a charging and discharging cycle(s)    t1: time of a charging or a discharging(s)    Ce: value of the capacitance Ce (F)    Vref+: second reference value (V)    Vref−: first reference value (V)    i: absolute value of the charging or discharging current (A)
The change from a charging C+ to a discharging C− (and vice-versa) is controlled by the control means 300 on the basis of the result of the comparisons between the voltage Vce and each of the two reference values (Vref−, Vref+).
Table 1 below shows the four possible configurations of the first SW1 and second SW2 switches, according to the values received on the R and S inputs and the value of the corresponding output Q. Qt−1 signifies that the output Q retains the value of the preceding moment of time. The table refers to four points a, b, c, d shown in FIG. 2:                point a: Vce>Vref−, the capacitance Ce is charging C+, the first switch SW1 is closed (state 1) and the second switch SW2 is open (state 0),        point b: Vce>Vref+ opening of the first switch SW1 (state 0) and closing of the second switch SW2 (state 1) in order to discharge the capacitance Ce,        point c: Vce<Vref+, the capacitance Ce is discharging C−, the same configuration as for the point b, the first switch SW1 is open and the second switch SW2 is closed,        point d: Vce<Vref−, opening of the second switch SW2 (state 0) and closing of the first switch SW1 (state 1) in order to charge the capacitance Ce.        
A voltage inverter INV (FIGS. 1 and 3) situated upstream of the second switch SW2 between the control means 300 and the second switch SW2 makes it possible to invert the value of the control signal SL at the output Q, which gives a signal of opposite value SL applied to this second switch SW2. This makes it possible for the first SW1 and second SW2 switches to receive opposite instructions (0, 1), corresponding to the values of table 1 (see SW1, and SW2).
The charging C+ and discharging C− cycle thus described is repeated a predetermined number Nc of times.
The output Q of the control means 300 is therefore alternately connected either to the power supply voltage Vdd (when the first switch SW1 is closed and the second switch SW2 is open), or to ground (when the second switch SW2 is closed and the first switch SW1 is open).
TABLE 1PointabcdRS2 = 0S1 = 1S2 = 0S2 = 0SS1 = 0S1 = 0S1 = 0S1 = 1Q (SL) Qt−1 (= 1)0Qt−1 (= 0) 1SW1 State 1001Q(SL)0110SW2 State0110
The output voltage Vc of the control means 300 is therefore equal to the power supply voltage Vdd during the charging C+ and is equal to 0 Volt during the discharging C− of the capacitance Ce.
A counter 400 measures the time tmes1 necessary for the measuring device D in order to carry out this predetermined number Nc of charging and discharging cycles.
Hence the equation (2):tmes1=Nc*T1
When the user moves his hand M towards the handle, the capacitance Ce increases by a value ΔCe (see FIG. 1). This variation ΔCe of capacitance has the effect of increasing the charging and discharging cycle time, and the new time of a cycle T2 (see FIG. 2, curve shown in dotted line), when the hand M is present, is longer than the cycle time T1 without the presence of the hand M. A new and longer time tmes2 is then necessary for carrying out the same predetermined number Nc of cycles (see FIG. 2) when the hand M is present close to the capacitive sensor.
The difference between the new time tmes2 and the previously measured time tmes1 is representative of the variation ΔCe of the capacitance Ce due to the presence of the hand M. According to the prior art, the variation ΔCe is given by the following equation (3):
      Δ    ⁢                  ⁢    Ce    =                    (                  t          ⁢                                          ⁢          mes          ⁢                                          ⁢          2          ⁢                                          ⁢          tmes          ⁢                                          ⁢          1                )            *      t              2      *              (                              Vref            +                    -                      Vref            -                          )            *      Nc      Since, according to the equation (2):tmes1=Nc*T1and also:tmes2=Nc*T2where:    ΔCe: variation of the capacitance Ce (F)    tmes2: new time (with the hand present) for carrying out a predetermined number Nc of charging and discharging cycles(s)    tmes1: time (without the hand present) for carrying out a predetermined number Nc of charging and discharging cycles(s)    Vref+: second reference value (V)    Vref−: first reference value (V)    i: absolute value of the charging or discharging current (A)    Nc: predetermined number of charging and discharging cyclesThe following equation (4) is therefore obtained:
      Δ    ⁢                  ⁢    Ce    =                    (                              T            ⁢                                                  ⁢            2                    -                      T            ⁢                                                  ⁢            1                          )            *      t              2      *              (                              Vref            +                    -                      Vref            -                          )                T1: time of a charging and discharging cycle (without hand present)(s)    T2: new time of a charging and discharging cycle (with hand present)(s)
The closer the hand M approaches, the greater this variation ΔCe becomes. When it exceeds a threshold, it is considered that the user wishes to access his vehicle and therefore the detection of presence of the user is validated.
The electrical consumption of the measuring device D and therefore of the capacitive sensor (or of the capacitance Ce electrode) is directly proportional to the time of a charging and discharging cycle T1 (or T2). If it is desired to lower this consumption, it is necessary to reduce the time of a cycle T1.
According to equation (1), in order to reduce the time of a cycle T1, it is necessary:                either to lower the value of the voltage difference ΔV between the second reference value and the first reference value, that is to say to lower:ΔV=Vref+−Vref−        
or to increase the value of the charging current i,
or to lower the value of the capacitance Ce,
now, according to equation (4):                lowering the value of the voltage difference ΔV=Vref+−Vref− increases the value of the variation ΔCe which is measurable, and therefore degrades the sensitivity of the sensor,        increasing the value of the charging current i also increases the value of the variation ΔCe which is measurable, and therefore degrades the sensitivity of the sensor,        the value of the capacitance Ce has no effect on the value of the variation ΔCe.        
Consequently, these traditional approaches of reducing the cycle time T1 have the effect of degrading the sensitivity of the sensor, which is not desirable.